Pressure-sensitive adhesive sheet

ABSTRACT

Provided is a PSA sheet capable of showing high long-term adhesive strength and good air release properties between the PSA sheet and an adherend. This invention provides a PSA sheet comprising a substrate film and a PSA layer provided to at least one face of the substrate film. The PSA sheet comprises a coating layer partially covering the PSA layer surface, whereby the PSA layer surface has a coating layer-bearing area and a coating layer-free area. The coating layer-free area occupies 70% or more of the PSA layer surface. In a top view of the PSA layer surface, the coating layer-bearing area has a linearly extending part running from one edge to another edge of the PSA layer. The linearly extending part has a width in a range of 0.1 mm to 2 mm.

TECHNICAL FIELD

The present invention relates to a pressure-sensitive adhesive sheet.

This application claims priority to Japanese Patent Application No. 2014-242183 filed on Nov. 28, 2014 and Japanese Patent Application No. 2015-164270 filed on Aug. 21, 2015; the entire contents thereof are incorporated herein by reference.

BACKGROUND ART

In general, pressure-sensitive adhesive (or PSA; the same applies hereinafter) has characteristics to be in a soft solid (viscoelastic) state in a room temperature range and easily adhere to adherend with some pressure. With the benefit of such properties, PSA is widely used in forms of substrate-supported PSA sheets having a PSA layer at least on one face of the substrate for purposes including fastening and surface protection of various articles and obtaining desirable appearances such as for decorative purposes. Documents disclosing these types of conventional art include Patent Document 1.

CITATION LIST Patent Literature

[Patent Document 1] Japanese Patent Application Publication No. 2000-160117

SUMMARY OF INVENTION Technical Problem

With respect to conventional PSA sheets, when the PSA sheets are adhered to adherends, there have been cases where a fluid substance such as air is left between the PSA sheets and the adherends to form bubbles and the like (trapped air, etc.), thereby causing degradation of the appearances. Such bubbles and the like are not desirable, either, in view of their negative impact on the adhesive properties such as reduced adhesive strength. To prevent formation of the bubbles and the like or to provide features (or air release properties) to eliminate the sort of bubbles if formed, a technique of partial lamination of a non-adhesive layer on the PSA layer surface is suggested (Patent Document 1). However, with respect to the art according to Patent Document 1, because the non-adhesive layer occupies a large portion of the PSA layer surface, the art has not been satisfactory in dealing with the influence on the adhesive properties, resulting in reduced adhesive strength, etc. The studies by the present inventors have revealed that it does not serve as a highly reliable bonding means particularly in an application that continues to require high adhesive strength after adhesion.

The present invention has been made in view of these circumstances with an objective to provide a PSA sheet capable of exhibiting high long-term adhesive strength as well as good air release properties between the PSA sheet and an adherend.

Solution to Problem

The present invention provides a PSA sheet comprising a substrate film and a PSA layer provided to at least one face of the substrate film. The PSA sheet comprises a coating layer that partially covers the PSA layer surface, whereby the PSA layer surface has a coating layer-bearing area and a coating layer-free area. The % surface area of the coating layer-free area in the PSA layer surface is 70% or higher. When the PSA layer surface is seen from the top, the coating layer-bearing area has a linearly extending part that runs from one edge to another edge of the PSA layer. The linearly extending part has a width in a range of 0.1 mm to 2 mm.

As a general tendency, with respect to a PSA sheet, with decreasing surface area of the adhesive face, the adhesiveness proportionally decreases as well. When a coating layer is placed on the adhesive face of the PSA sheet, the exposed surface area of the adhesive face will decrease by as much as the portion covered by the coating layer; and therefore, a similar tendency may be seen. In fact, in the working examples described later, with increasing surface area of the coating layer and decreasing exposed surface area of the PSA layer in the PSA layer surface, the initial adhesive strength immediately after application decreased in a proportional manner. However, studies by the present inventors have revealed that the long-term adhesive strength shows a tendency different from the initial adhesive strength. In particular, it has been found that, in the embodiment where the coating layer is partially placed on the PSA layer surface, when the % surface area of the coating layer-free area in the PSA layer surface was less than 70%, the adhesive strength hardly increased from the level immediately after it was applied; and there was a higher tendency for insufficient long-term adhesive strength as compared to when the % surface area was 70% or higher (see Examples described later). In the art disclosed herein, high long-term adhesive strength is obtained when the coating layer-free area occupies 70% or more surface area. Naturally, however, with increasing surface area of the coating layer-free area, the surface area of the coating layer-bearing area decreases in relation. Thus, the primary effect of the coating layer to provide air release properties to the interface between the PSA sheet and an adherend may quite possibly be impaired. Thus, in the art disclosed herein, the coating layer is placed so that, when the PSA layer surface is seen from the top, it has a linearly extending part that runs from one edge to another edge of the PSA layer; and the width of the linearly extending part is set in the range of 0.1 mm to 2 mm. By this, in the PSA sheet capable of showing high long-term adhesive strength, good air release properties can be realized. The linearly extending part has a width of preferably 0.2 mm or greater (e.g. 0 3 mm or greater, or even 0.5 mm or greater), and preferably 1.2 mm or less (more preferably 0.7 mm or less, or yet more preferably 0.4 mm or less). Herein, the long-term adhesive strength refers to the adhesive strength at 24 hours after the PSA sheet is applied to an adherend.

In a preferable embodiment of the PSA sheet disclosed herein, the PSA layer surface is provided with a plurality of the linearly extending parts. Among the plurality of the linearly extending parts, two or more linearly extending parts in a group are placed at intervals arranged in the width direction, whereby the coating layer-bearing area has a stripe pattern. In such an embodiment, through the two or more linearly extending parts in the stripe pattern, air and the like to remain between the adherend surface and the adhesive face are efficiently eliminated.

In a preferable embodiment of the PSA sheet disclosed herein, the coating layer-bearing area comprises a first stripe pattern and a second stripe pattern that is placed to intersect the first stripe pattern, whereby the coating layer-bearing area has a lattice pattern. In such an embodiment, air and the like to remain between the adherend surface and the adhesive face are efficiently eliminated.

In a preferable embodiment of the PSA sheet disclosed herein, in the coating layer-bearing area, the two or more linearly extending parts in the stripe pattern have intervals in a range of 1.0 mm to 10 mm. In such an embodiment, high long-term adhesive strength and good air release properties are combined in a well-balanced manner. The intervals between the linearly extending parts are more preferably 1 5 mm to 8 mm, or yet more preferably 2.5 mm to 5 mm.

In a preferable embodiment, the PSA sheet disclosed herein has an adhesive surface on which the coating layer is partially placed and that exhibits a 24-hour 180° peel strength of 13 N/25 mm or greater. According to the art disclosed herein, in the embodiment where the coating layer is provided, such peel strength can be obtained.

The present invention also provides a release liner-backed PSA sheet comprising a PSA sheet disclosed herein and a release liner that protects the PSA layer surface in the PSA sheet. Of the surfaces of the release liner, the PSA layer-side surface is formed smooth. In such an embodiment, greater adhesive properties tend to be obtained. According to the art disclosed herein, in this embodiment, when the release liner is used as a coating layer-transferring film, a good transfer of the coating layer can be obtained.

The present invention also provides a release liner for PSA sheets, the release liner comprising a releasable support having at least one releasable face. The releasable face of the releasable support in the release liner is provided with a coating layer that can be transferred onto a PSA sheet, whereby the releasable face has a coating layer-bearing area and a coating layer-free area. The % surface area of the coating layer-free area in the releasable face is 70% or greater. The coating layer further has a linearly extending part that runs from one edge to another edge of the releasable face. The linearly extending part has a width in a range of 0.1 mm to 2 mm. By transferring the coating layer onto the PSA layer surface of the PSA sheet using such a release liner, it is possible to preferably make a PSA sheet that combines high long-term adhesive strength and good air release properties at a high level.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic top view of the PSA sheet according to an embodiment.

FIG. 2 shows a cross-sectional diagram at line II-II in FIG. 1.

FIG. 3 shows a schematic cross-sectional diagram of the release liner-backed PSA sheet according to an embodiment.

FIG. 4 shows a schematic cross-sectional diagram of the release liner-backed PSA sheet according to another embodiment.

FIG. 5 shows a schematic cross-sectional diagram of the release liner for the PSA sheet according to an embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention are described below. Matters necessary to practice this invention other than those specifically referred to in this description may be comprehended by a person of ordinary skill in the art based on the instruction regarding implementations of the invention according to this description and the common technical knowledge in the pertinent field. The present invention can be practiced based on the contents disclosed in this description and common technical knowledge in the subject field. In the drawings referenced below, a common reference numeral may be assigned to members or sites producing the same effects, and duplicated descriptions are sometimes omitted or simplified. The embodiments described in the drawings are schematized for clear illustration of the present invention, and do not necessarily represent the accurate sizes or reduction scales of the PSA sheet of the present invention provided as an actual product.

The concept of PSA sheet herein encompasses so-called PSA tapes, PSA labels, and PSA films The PSA sheet disclosed herein can be in a roll form or in a flat sheet form. Alternatively, the sheet may be further processed into various forms.

FIG. 1 shows a schematic top view of the PSA sheet according to an embodiment. FIG. 2 shows a cross-sectional diagram at line II-II in FIG. 1. With reference to the drawings, the PSA sheet in this embodiment is described.

As shown in FIGS. 1 and 2, PSA sheet 1 according to this embodiment has a layered structure formed of a substrate film 10 and a PSA layer 20. Substrate film 10 supports the PSA layer 20. In PSA sheet 1, the surface 20A on the PSA layer 20 side forms an adhesive face 1A. The other face 1B (the surface on the film substrate 10 side) of PSA sheet 1 is a non-adhesive face.

On the surface 20A of the PSA layer 20, a coating layer 30 is partially placed. In other words, the PSA layer surface 20A is partially covered with the coating layer 30, and has a coating layer-bearing area 50 where the coating layer 30 is placed and a coating layer-free area 51 where the coating layer 30 is not placed and the PSA layer 20 is exposed on the surface. When the PSA sheet 1 is applied to an adherend, the coating layer 30 forms pathways for air and the like at the interface between the PSA sheet 1 and the adherend, thereby providing air release properties. The surface of the coating layer 30 may be flush with the PSA layer surface 20A or may protrude at least partially in the thickness direction from the PSA layer surface 20A.

When the PSA layer surface 20A is seen from the top, the coating layer-bearing area 50 is in a certain pattern (coating layer pattern) 60. In the present embodiment, the coating layer-bearing area 50 shows a lattice pattern 60. In particular, when the PSA layer surface 20A is seen from the top, the lattice pattern 60 of the coating layer 30 is formed of a first stripe pattern 70 and a second stripe pattern 80 that is placed to intersect with the first stripe pattern 70.

The first stripe pattern 70 is formed of a plurality of long parts (bands) 75A, 75B and 75C that extend straight from one edge to another edge of the PSA layer 20. The plurality of the long parts 75A, 75B and 75C are placed in parallel at intervals arranged in the width direction. In this embodiment, the long parts 75A, 75B and 75C are placed at an angle such that their length directions intersect the edges (ends, limits) of the width direction of the PSA sheet 1, with each long part reaching two edges (two sides) of the PSA layer 20. Herein, the term “long part” means a part having a length direction and a width direction (a direction perpendicularly intersecting the length direction) (i.e. a part having a length and a width), typically referring to a long band regardless of whether it is straight or curved. In the other words, the long part can be described as a linearly extending part.

Similarly to the first stripe pattern 70, the second stripe pattern 80 is also formed of a plurality of long parts (bands) 85A, 85B and 85C that extend straight from one edge to another edge of the PSA layer 20. The plurality of the long parts 85A, 85B and 85C are placed in parallel at intervals arranged in the width direction. In this embodiment, the long parts 85A, 85B and 85C are placed at an angle such that their length directions intersect the edges (ends, limits) of the width direction of the PSA sheet 1, with each long part reaching two edges (two sides) of the PSA layer 20. In this embodiment, the long parts 75A, 75B, 75C, 85A, 85B and 85C are straight, but are not limited thereto. Each long part may be curvilinear. In such an embodiment, a plurality of the long parts may form a wavy stripe pattern, etc.

In this embodiment, the first stripe pattern 70 and the second stripe pattern 80 intersect each other so that the long parts 75A, 75B and 75C of the first stripe pattern 70 and the long parts 85A, 85B and 85C of the second stripe pattern 80 cross one another almost perpendicularly. Thus, the long parts 75A, 75B and 75C of the first stripe pattern 70 partially overlap the long parts 85A, 85B and 85C of the second stripe pattern 80.

Herein, the lattice pattern typically refers to a pattern that includes two stripe patterns intersecting each other and encompasses not only a rhombic lattice as in the present embodiment, but also various lattice shapes such as a square lattice and a triangular lattice. When the long parts are straight, the angle (the acute angle) at an intersection of the two stripe patterns can be in a range from 10° to 90° (preferably 45° to 90°, typically 60° to 90°). The lattice pattern disclosed herein also encompasses a pattern that includes a stripe pattern formed of several linearly extending parts with repeated bending, for instance, a pattern such as a hexagonal lattice. In such a pattern, adjacent long parts may be partially connected to one another. From the standpoint of the air release properties, the coating layer-bearing area preferably comprises one, two or more stripe patterns. Thus, the coating layer-bearing area (typically in a lattice pattern) may include a third stripe pattern.

In the coating layer-bearing area 50, the widths (W1) of the respective long parts 75A, 75B, 75C, 85A, 85B and 85C are in a range of 0.1 mm to 2 mm. This can combine high long-term adhesive strength (adhesive strength after aged) and good air release properties. From the standpoint of enhancing the air release properties, the widths W1 of the long parts are preferably 0.2 mm or greater, more preferably 0.3 mm or greater, or yet more preferably 0.5 mm or greater. From the standpoint of the adhesive strength, the appearance, etc., the widths W1 of the long parts are preferably 12 mm or less, more preferably 1.0 mm or less, yet more preferably 0.7 mm or less, particularly preferably 0.5 mm or less, or most preferably 0.4 mm or less.

In the coating layer-bearing area 50, the intervals W2 between the long parts 75A, 75B and 75C forming the first stripe pattern 70 are preferably selected from a range of 1.0 mm to 10 mm. By this, in an embodiment where the widths of the long parts are in the range, there is a higher tendency that high long-term adhesive strength is combined with air release properties in a well-balanced manner. Here, the intervals W2 between the long parts refer to the widths of spaces present between any two adjacent long parts in the PSA layer surface. From the standpoint of increasing the long-term adhesive strength, etc., the intervals W2 between the long parts are more preferably 1.5 mm or greater, or yet more preferably 2.5 mm or greater. The intervals W2 between the long parts can be about 8 mm or less (e.g. 5 mm or less, typically 3 mm or less). The intervals between the long parts forming the second stripe pattern 80 can also be preferably selected from the ranges exemplified for the intervals between the long parts 75A, 75B and 75C. The intervals W2 are preferably evenly spaced.

From the standpoint of combining well-balanced high long-term adhesive strength and air release properties, the pitch of the long parts is preferably in a range of 1 mm to 20 mm. The pitch of the long parts is more preferably 1 5 mm or greater, yet more preferably 2 mm or greater (e.g. 2.5 mm or greater); it is more preferably 15 mm or less (e.g. 12 mm or less), or yet more preferably 5 mm or less. The pitch refers to the distance (interval) between the centerlines of the width directions (i.e. the lengthwise centerlines) of the long parts.

Before used, as shown in FIG. 3, the PSA sheet 1 may be in a form of a release liner-backed PSA sheet 1 where the PSA layer 20 is protected with a release liner 40 having a release face at least on the PSA layer surface 20A side. Alternatively, it may be in a form such that the back face (opposite from the PSA layer 20 side surface) of substrate film 10 is a release face and the PSA sheet 1 is wound so that the back face is brought into contact with the PSA layer 20 whereby the PSA layer 20 is protected with the back face of the substrate film. Such an adhesively single-faced PSA sheet (single-faced PSA sheet) having only one adhesive face is favorable, for instance, when the surface opposite from the adhesive surface requires features such as decoration and surface protection, or when it is used as a paint substitute sheet.

When the PSA sheet disclosed herein is an adhesively double-faced substrate-backed PSA sheet (a double-faced PSA sheet) as shown in FIG. 4, the PSA sheet 2 may be in an embodiment such that the respective faces (both non-releasable) of substrate film 10 are provided with PSA layers 21 and 22 with the PSA layers 21 and 22 protected with release liners 41 and 42, respectively, with each liner having a release face at least on the PSA layer side. In the PSA sheet 2, a coating layer 30 is partially placed only on the surface of the PSA layer 21, and no coating layer is formed on the PSA layer 22. Alternatively, although not specifically shown in a drawing, the double-faced PSA sheet may be in an embodiment such that PSA layers are provided to the respective faces (both non-releasable) of the substrate film and one of the PSA layers is protected with a release liner having a release face on each side. By winding the PSA sheet so that the other PSA layer is brought into contact with the back face of the release liner, this type of PSA sheet can be made into an embodiment where the two PSA layers are protected with the one release liner. The double-faced PSA sheet is preferably used, for instance, for bonding/fixing applications.

<Properties of PSA Sheet, etc.>

The PSA sheet disclosed herein is characterized by the % surface area of the coating layer-free area in the PSA layer surface (which can be the % surface area of the coating layer in the adhesive face (adhesive surface) of the PSA sheet) being 70% or higher. This can ensure high long-term adhesive strength. The % surface area is preferably 75% or higher, or more preferably 80% or higher; from the standpoint of obtaining good air release properties, the % surface area is preferably 90% or lower, or more preferably 85% or lower.

In a preferable embodiment, the adhesive surface (typically formed of the PSA layer surface and the coating layer surface) of the PSA sheet shows a 24-hour 180° adhesive strength (180° adhesive strength after 24-hour adhesion, or a 24-hour adhesion strength, hereinafter) of 13 N/25 mm or greater. The PSA sheet can exhibit at least a certain level of long-term adhesive strength (adhesive strength after aged) while having good air release properties. The 24-hour adhesion strength is preferably 14 N/25 mm or greater (e.g. 16 N/25 mm or greater). The 24-hour adhesion strength can be determined by the method described next. In particular, the PSA sheet is cut to a 25 mm wide by 100 mm long size to obtain a measurement sample; and in an environment at 23° C. and 50% RH, the adhesive surface of the measurement sample is press-bonded to the surface of a stainless steel plate (SUS 304BA plate) with a 2 kg roller moved back and forth once. This is left standing in the same environment for 24 hours. Subsequently, using a universal tensile/compression tester, based on JIS Z 0237:2000, the peel strength (N/25 mm) is determined at a tensile speed of 300 mm/min at a peel angle of 180°. More specifically, it is determined by the method described later in Examples.

In a preferable embodiment, the PSA sheet has transparency (including semi-transparency). In such a PSA sheet, when bubbles and the like are trapped between the PSA sheet and an adherend, they are visible through the PSA sheet and are likely to degrade the appearance. The art disclosed herein prevents formation of the sort of bubbles between the PSA sheet and the adherend; and therefore, an excellent appearance can be obtained in a transparent PSA sheet. In particular, the PSA sheet may exhibit a total light transmittance of 80% or higher (e.g. 90% or higher, typically 95% or higher). The PSA sheet preferably has a haze value of 10% or lower (e.g. 5% or lower). The total light transmittance and the haze value can be determined using a commercial transmissometer (e.g. product name HAZE METER HM-150 available from Murakami Color Research Laboratory). The total light transmittance and the haze value of the substrate film described later are also determined by the same methods.

The overall thickness of the PSA sheet disclosed herein (including the PSA layer and the substrate, but excluding the release liner) is not particularly limited. It is suitably in a range of about 2 μm to 1000 μm, or preferably 5 μm to 500 μm (e.g. 10 μm to 300 μm, typically 30 μm to 100 μm). The PSA sheet limited in overall thickness can be advantageous in view of making products to which the PSA sheet is applied smaller, lighter, resource-saving, and so on.

<Substrate Film>

Examples of the substrate film disclosed herein include resin film, paper, cloth, rubber film, foam film, and metal foil as well as a composite and a laminate of these. In particular, from the standpoint of the ease of application and the appearance, it preferably comprises a resin film layer. The inclusion of the resin film layer is advantageous also from the standpoint of the size stability, the accuracy of thickness, the ease of processing, the tensile strength and so on. Examples of the resin film include polyolefinic resin film such as polyethylene (PE), polypropylene (PP), and ethylene/polypropylene copolymers; polyester-based resin film such as polyethylene terephthalate (PET), polybutylene terephthalate, and polyethylene naphthalate; vinyl chloride-based resin film; vinyl acetate-based resin film; polyimide-based resin film; polyamide-based resin film; fluororesin film; and cellophane. Favorable examples include resin films formed from PE, PP and PET. Among the resin films, polyester film is more preferable; among them, PET film is even more preferable. The substrate film may have a monolayer structure or a multilayer structure formed of two, three or more layers.

In a preferable embodiment, the substrate film is a substrate comprising a foam film (a foam-containing substrate). This provides impact-absorbing capabilities to the PSA sheet. Here, the foam film refers to a film structure having a part with foam cells (a foam cell structure). The foam-containing substrate may be a mono-layer structure formed from a foam film or a multi-layer structure wherein at least one of whose two or more layers is formed of a foam film (a foam layer). A configurational example of the foam-containing substrate is a composite substrate in which a foam film (a foam layer) and a non-foamed film (a non-foamed layer) are laminated. The non-foamed film (non-foamed layer) refers to a film structure that has not been subjected to a purposeful foaming process (e.g. a process to incorporate foam cells), referring to a film essentially free of a foam cell structure. Atypical example of the foam film is a resin film (e.g. a polyester-based resin film such as of PET) having an expansion rate of less than 1.1-fold (e.g. less than 1.05-fold, typically less than 1.01-fold). When the substrate film comprises two or more foam layers, the materials and structures of these foam layers can be identical or different. When the foam film has a multi-layer structure that includes a foam layer, from the standpoint of increasing the tightness between layers, adhesive layers may be placed between the layers.

The foam film is not particularly limited in average foam cell diameter; it is usually suitably 10 μm to 200 μm, preferably 20 μm to 180 μm, or more preferably 30 μm to 150 μm. When the average foam cell diameter is10 μm or larger, the impact-absorbing properties tend to increase. On the other hand, when the average foam cell diameter is 200 μm or smaller, the handling properties and waterproof properties (water-blocking properties) tend to increase.

The average foam cell diameter (μm) of the foam film can be determined, using a low-vacuum scanning electron microscope to take an enlarged image of a cross section of the foam and subjecting it to image analysis. About 20 to 30 foam cells can be analyzed. As the low-vacuum scanning electron microscope, for instance, product name S-3400N Scanning Electron Microscope available from Hitachi High-Tech Science Systems Corporation) can be used.

The foam film is not particularly limited in density (apparent density); it is usually suitably 0.01 g/cm³ or higher, preferably 0.01 g/cm³ to 0.7 g/cm³, or more preferably 0.02 g/cm³ to 0.5 g/cm³. When the density is 0.01 g/cm³ or higher, the strength of the foam film (and even that of the PSA sheet) will increase with a tendency toward greater impact resistance and handling properties. On the other hand, when the density is 0.7 g/cm³ or lower, the conformability to a difference in level tends to increase without an excessive decrease in flexibility.

The density (apparent density) of the foam film is determined based on the method described in JIS K 7222:1999. In particular, the foam film is punched out into a 100 mm by 100 mm size to prepare a specimen and the dimensions of the specimen are measured. Using a 1/100 dial gauge with a 20 mm diameter measurement terminal, the thickness of the specimen is measured. From these values, the volume of the foam film specimen is determined The specimen is weighed on a top-loading balance (minimum scale 0.01 g or greater). From these values, the apparent density (g/cm³) of the foam film can be determined

The 50% compressive stress of the foam film is not particularly limited. From the standpoint of the impact resistance, the foam film suitably shows a 50% compressive stress of 0.1 N/cm² or greater. When the 50% compressive stress is at or above a certain value, for instance, even if the foam film is thin (e.g. about 100 μm thick), it can show sufficient resistance when compressed (resilience to compression) and maintain good impact resistance. The 50% compressive stress is preferably 0.2 N/cm² or greater, or more preferably 0.5 N/cm² or greater. From the standpoint of combining flexibility and impact resistance in a well-balanced way, the 50% compressive stress is suitably 8 N/cm² or less, preferably 6 N/cm² or less, or more preferably 3 N/cm² or less.

The 50% compressive stress (hardness) of the foam film is determined based on JIS K 6767:1999. In particular, the foam film is cut to 100 mm by 100 mm pieces. These pieces are layered to a total thickness of at least 2 mm and the resultant is used as a measurement sample. At room temperature, using a compression tester, the measurement sample is compressed at a rate of 10 mm/min. When compressed to 50% (when compressed to 50% of its initial thickness) and held at 50% compression for 10 seconds, the value (resilience in N/cm²) is recorded as the 50% compressive stress. Other conditions (e.g. jig and calculation method, etc.) are conformed to JIS K 6767:1999.

The foam constituting the foam film disclosed herein is not particularly limited in foam cell structure. The foam cell structure can be a continuous foam cell structure, an isolated foam cell structure, or a semi-continuous foam cell structure. From the standpoint of the impact absorbing properties, continuous and semi-continuous foam cell structures are preferable.

The material of the foam film is not particularly limited. The foam film can be typically formed from a material comprising a polymer component (e.g. a thermoplastic polymer). A preferable foam film is usually formed of foam of a plastic material (plastic foam). The plastic material (which means to include a rubber material) for forming the plastic foam is not particularly limited; a suitable species can be selected among known plastic materials. For the plastic material (typically a thermoplastic polymer), solely one species or a combination of two or more species can be used. The primary component (typically a component accounting for more than 50% by weight) among the polymers in the substrate film or the foam film may be referred to as the “base polymer” hereinafter.

Specific examples of the foam include polyolefinic resin foam such as PE foam and PP foam; polyester-based foam such as PET foam, polyethylene naphthalate foam and polybutylene terephthalate foam; polyvinyl chloride-based resin foam such as polyvinyl chloride foam; vinyl acetate-based foam; acrylic resin foam; polyphenylene sulfide resin foam; amide-based resin foam such as polyamide (nylon) resin foam and all-aromatic polyamide (aramide) resin foam; polyimide-based resin foam; polyether ether ketone (PEEK) foam; styrene-based resin foam such as polystyrene foam; and urethane-based resin foam such as polyurethane resin foam. As the foam, rubber-based resin foam such as polychloroprene rubber foam can be used as well.

In a preferable embodiment, acrylic resin foam (foam formed from acrylic resin) is used as the foam. Here, the acrylic resin foam refers to foam comprising an acrylic polymer as the base polymer. The acrylic polymer in this description is as defined later. As the alkyl (meth)acrylate forming the acrylic polymer, one, two or more species can be preferably used among alkyl (meth)acrylates having acyclic alkyl groups with 1 to 20 (preferably 1 to 8, typically 1 to 4) carbon atoms. Preferable examples of the alkyl (meth)acrylate include ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate. The amount of the alkyl (meth)acrylate as the primary monomer is suitably 70% by weight or more of all monomers in the acrylic polymer, or preferably 75% by weight or more (e.g. 80% by weight or more). The amount of the alkyl (meth)acrylate is suitably 98% by weight or less of all the monomers, or preferably 97% by weight or less (e.g. 96% by weight or less).

The secondary monomer co-polymerizable with the alkyl (meth)acrylate as the primary monomer may be useful in introducing crosslinking points in the acrylic polymer or in increasing the cohesive strength of the acrylic polymer. As the secondary monomer, one, two or more species of functional group-containing monomers can be used among, for instance, carboxy group-containing monomers, hydroxy group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, cyano group-containing monomers, monomers having nitrogen atom-containing rings and the like. The secondary monomer can also be a vinyl ester-based monomer such as vinyl acetate, an aromatic vinyl compound such as styrene, a sulfonate group-containing monomer, a phosphate group-containing monomer and the like. The amount of the secondary monomer is suitably 0.5% by weight or more of all monomers in the acrylic polymer, or preferably 1% by weight or more. The amount of the secondary monomer is suitably 30% by weight or less of all the monomers, or preferably 10% by weight or less.

When the foam is formed with an emulsion-based resin composition by a foaming method where gases including air are mixed in mechanically such as by stirring, it is preferable that the monomers forming the acrylic polymer comprise a nitrogen atom-containing monomer as the secondary monomer. This facilitates the formation of foam cells in the foaming process and may increase the viscosity of the composition when forming the foam (typically when drying the resin composition), whereby the foam cells are readily kept in the foam body.

Examples of the nitrogen atom-containing monomer include cyano group-containing monomers such as acrylonitrile and methacrylonitrile; lactam ring-containing monomers such as N-vinyl-2-pyrolidone; amide group-containing monomers such as (meth)acrylamide, N-hydroxyethyl(meth)acrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide and diacetone acrylamide. These can be used solely as one species or in a combination of two or more species. Among them, cyano group-containing monomers such as acrylonitrile and lactam ring-containing monomers such as N-vinyl-2-pyrolidone are preferable.

The amount of the nitrogen atom-containing monomer is suitably 2% by weight or more of all monomers in the acrylic polymer, or preferably 3% by weight or more (e.g. 4% by weight or more). The amount of the nitrogen atom-containing monomer is suitably 30% by weight or less of all the monomers, or preferably 25% by weight or less (e.g. 20% by weight or less).

The method for obtaining the acrylic polymer is not particularly limited. Various polymerization methods known as procedures for the synthesis of acrylic polymer can be suitably used, such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, active energy ray polymerization (e.g. UV polymerization). For instance, a desirable acrylic polymer can be obtained by dissolving or dispersing a monomer mixture in a suitable polymerization solvent (toluene, ethyl acetate, water, etc.) and carrying out polymerization using a polymerization initiator such as an azo-based polymerization initiator and a peroxide-based initiator. In view of the ease of foaming and environmental aspects, it is preferable to use acrylic resin foam (emulsion-based acrylic resin foam) obtained by emulsion polymerization.

From the standpoint of increasing the cohesive strength, the acrylic resin foam-forming composition preferably comprises a crosslinking agent. The type of crosslinking agent is not particularly limited. Among various crosslinking agents, one, two or more species can be suitably selected and used. Favorable examples of the crosslinking agent include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, carbodiimide-based crosslinking agents, melamine-based crosslinking agents and metal oxide-based crosslinking agents. In particular, oxazoline-based crosslinking agents are preferable. The amount of the crosslinking agent used is not particularly limited. To 100 parts by weight of the acrylic polymer, it is suitably selected from a range of about 10 parts by weight or less (e.g. about 0.005 part to 10 parts by weight, preferably about 0.01 part to 5 parts by weight).

In another preferable embodiment, polyolefinic resin foam (resin foam formed from a polyolefin) is used as the foam. As the plastic material forming the polyolefinic foam, various known or commonly-used polyolefinic resins can be used without particular limitations. Examples include polyethylene such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and metallocene catalyst-based linear low density polyethylene; polypropylene; ethylene-propylene copolymer; and ethylene-vinyl acetate copolymer. Among these polyolefinic resins, solely one species or a combination of two or more species can be used.

From the standpoint of the impact resistance, waterproof properties, etc., favorable examples of the foam film in the art disclosed herein include a polyethylene-based foam film essentially formed of polyethylene-based resin foam and a polypropylene-based foam film essentially formed of polypropylene-based resin foam. Here, the polyethylene-based resin refers to resin formed from ethylene as the primary monomer (i.e. the primary component among the monomers) and may include HDPE, LDPE and LLDPE as well as ethylene-propylene and ethylene-vinyl acetate copolymers of which ethylene is copolymerized at a ratio above 50% by weight. Similarly, the polypropylene-based resin refers to resin formed from propylene as the primary monomer. As the foam film in the art disclosed herein, a polypropylene-based foam film can be preferably used.

The foaming method for the foam film is not particularly limited. In accordance with the purpose, ease of procedures, etc., chemical procedures, physical procedures and so on can be employed individually or in combination. From the standpoint of the contamination, etc., physical foaming methods are preferable. Specific examples include a foaming method where a film-forming material is prepared to contain a foaming agent such as a low boiling compound (e.g. a hydrocarbon) and thermally expandable microspheres and foam cells are formed from the foaming agent, a foaming method where gases such as air are mechanically mixed in, a foaming method by solvent removal which takes advantage of removal of a solvent such as water, and a foaming method using a supercritical fluid. For instance, a method where an inert gas (e.g. carbon dioxide) is injected into the foam film-forming polymer under increased pressure and the resultant is placed under reduced pressure to form a foam film By this method, the average foam cell diameter can be easily controlled to be at or below a certain value and the foam film can be easily made to have a lower density.

The foam film is fabricated by employing a foaming method as described above. The formation of the foam film is not particularly limited. For instance, when employing a foaming method that mechanically admixes gases such as air, a resin composition (e.g. an emulsion-based resin composition) containing foam can be subsequently applied over a substrate or release paper, etc., and allowed to dry to obtain a foam film. From the standpoint of the foam stability, etc., the drying preferably includes a preliminary drying step at or above 50° C., but below 125° C. as well as a main drying step at 125° C. to 200° C. Alternatively, foam can be formed continuously into a film using a calender, extruder, conveyer belt casting and so forth; or a method where a kneaded mixture of foam-forming materials is foamed and molded in a batch process can be employed. In forming the foam film, a surface layer may be removed by slicing to adjust the film to obtain desirable thickness and foam characteristics.

The thermoplastic polymer (e.g. a polyolefinic polymer) that can be included in the foam film may comprise a thermoplastic elastomer that exhibits properties of rubber at room temperature, but shows thermoplasticity at a high temperature. From the standpoint of the flexibility and conformability, one, two or more species can be used among thermoplastic elastomers, for instance, olefinic elastomers such as ethylene-propylene copolymer, ethylene-propylene-diene copolymer, ethylene-vinyl acetate copolymer, polybutene, polyisobutylene, and chlorinated polyethylene; styrene-based elastomers such as styrene-butadiene-styrene copolymer; thermoplastic polyester-based elastomers; thermoplastic polyurethane-based elastomers; and thermoplastic acrylic elastomers. Among them, a thermoplastic elastomer having a glass transition temperature of room temperature or lower (e.g. 20° C. or lower). The thermoplastic elastomer content in the foam film is preferably about 10% to 90% by weight (e.g. 20% to 80% by weight) of the thermoplastic polymer in the foam film.

From the standpoint of the ease of mixing a foam-forming gas and the foam stability, as the foaming agent, various surfactants can be used in the foam film-forming material (e.g. an emulsion-based acrylic resin composition), with examples including anionic surfactants, cationic surfactants, nonionic surfactants and amphoteric surfactants. Hydrocarbon-based and fluorine-based surfactants can be used as well. In particular, from the standpoint of reducing the foam cell diameters and stabilizing the foam, anionic surfactants are preferable; ammonium salts of fatty acids (typically ammonium salts of higher fatty acids) such as ammonium stearate are more preferable. For the surfactant, solely one species or a combination of two or more species can be used. The surfactant content is preferably about 0.1 part to 10 parts by weight (e.g. 0.5 part to 8 parts by weight) to 100 parts by weight of the base polymer of the foam film The foaming agent in this description includes not only an agent that shows foaming capabilities, but also a foam cell diameter-adjusting agent to reduce the foam diameters as well as a foam stabilizer such as a foam-adjusting agent.

When the foam film-forming material is an aqueous dispersion (e.g. an acrylic emulsion), it is preferable to use a silicone-based compound as the foaming agent. By this, the recovery of thickness (the degree and speed of recovery) after compression tends to improve. A preferable silicone-based compound has 2000 or fewer siloxane bonds. Examples of the silicone-based compound include silicone oil, modified silicone oil, and silicone resin. In particular, dimethyl silicone oil and methyl phenyl silicone oil are preferable. As the silicone-based compound, a silicone-modified polymer (e.g. a silicone-modified acrylic polymer, a silicone-modified urethane-based polymer, etc.) can be used as well. These can be used solely as one species or in a combination of two or more species. The silicone compound content is preferably about 0.01 part to 5 parts by weight (e.g. 0.05 part to 4 parts by weight, typically 0.1 part to 3 parts by weight) to 100 parts by weight of the base polymer of the foam film.

From the standpoint of stabilizing the foam and increasing the ease of film formation, the foam film-forming material (e.g. an emulsion-based acrylic resin composition) may comprise a thickener. The thickener is not particularly limited. Examples include acrylic acid-based thickeners, urethane-based thickeners and polyvinyl alcohol-based thickeners. In particular, polyacrylic acid-based thickeners and urethane-based thickeners are preferable. The thickener content is preferably about 0.1 part to 10 parts by weight (e.g. 0.1 part to 5 parts by weight) to 100 parts by weight of the base polymer of the foam film.

When a foam-containing substrate is used as the substrate film, the foam film preferably comprises a foam-nucleating agent such as a metal hydroxide (e.g. magnesium hydroxide). This tends to facilitate the adjustment of the average foam cell diameter in the foam film to obtain desirable impact-absorbing properties, flexibility and so on. The foam-nucleating agent can be a metal oxide, composite oxide, metal carbonate, metal sulfate, etc. The foam-nucleating agent content is preferably about 0.5 part to 125 parts by weight (e.g. 1 part to 120 parts by weight) to 100 parts by weight of the base polymer of the foam film.

When using a foam-containing substrate as the substrate film, from the standpoint of inhibiting the foam from degassing while foam cells are being formed, the foam film preferably comprises a degassing inhibitor such as fatty acid amides. A more preferable fatty acid amide has a bis-amide structure. The degassing inhibitor can be a metal salt of a fatty acid as well. The degassing inhibitor content is preferably about 0.5 part to 10 parts by weight (e.g. 0.7 part to 8 parts by weight, typically 1 part to 6 parts by weight) to 100 parts by weight of the base polymer of the foam film.

The substrate film (e.g. a foam film) may comprise a softener so as to provide desirable fluidity to the film-forming material thereby to improve properties such as flexibility. With the inclusion of a softener in the foam film, properties such as ease of stretching the film and expansion ratio can be preferably adjusted. For example, one, two or more species can be preferably used among hydrocarbon-based softeners such as liquid paraffin, paraffin wax, micro wax and polyethylene wax; ester-based softeners such as glyceryl stearate; and fatty acid-based softeners. The softener content is preferably 0.5 part to 50 parts by weight (e.g. 0.8 part to 40 parts by weight, typically 1 part to 30 parts by weight) to 100 parts by weight of the base polymer of the substrate film (e.g. a foam film)

When emulsion-based acrylic resin foam is used, an arbitrary anticorrosive may be included to prevent corrosion of metal parts adjacent to the foam film. As the anticorrosive, an azole ring-containing compound is preferable. With the use of an azole ring-containing compound, inhibition of metal corrosion and tight adhesion to adherends can be combined at a high level. In particular, a compound with the azole ring forming a fused ring with an aromatic ring such as a benzene ring is preferable; benzotriazole-based compounds and benzothiazole -based compounds are especially preferable. The anticorrosive content is preferably about 0.2 part to 5 parts by weight (e.g. 0.3 part to 2 parts by weight) to 100 parts by weight of the base polymer of the foam film

In a preferable embodiment, the substrate film has transparency (including semi-transparency). In the PSA sheet comprising such a substrate film, when bubbles and the like are trapped between the PSA sheet and an adherend, they are visible through the PSA sheet and are likely to degrade the appearance. The art disclosed herein prevents formation of the sort of bubbles between the PSA sheet and the adherend; and therefore, an excellent appearance can be obtained in an embodiment comprising a transparent substrate. In particular, the substrate film may show a total light transmittance of 80% or higher (e.g. 90% or higher, typically 95% or higher). The substrate film preferably has a haze value of 10% or lower (e.g. 5% or lower).

To obtain desirable designs and optical properties, the substrate film (e.g. a resin film) may be colored black, white or other with various types of colorant (e.g. pigment) content. As a black colorant, carbon black is preferable. It is also possible to employ a method where at least one surface (one or each face) of the substrate film is subjected to printing to overlay one, two or more colored layers (e.g. a black layer and a white layer).

To the substrate film (e.g. a resin substrate film, a foam substrate film), various additives may be added as necessary, such as filler (inorganic filler, organic filler, etc.), anti-aging agent, antioxidant, UV ray absorber, antistatic agent, slip agent and plasticizer.

When the PSA sheet is adhesive on one face, between the two surfaces of the substrate film, the surface (back face) opposite from the surface to be provided with a PSA layer is preferably made smooth. The smooth surface may be the outer face of the PSA sheet; and therefore, when the PSA sheet having the smooth surface is used as, for instance, a decorative sheet or a surface protection sheet, it may provide a better appearance (design). In a preferable embodiment, from the standpoint of the adhesive properties and the quality of appearance (design), the back face of the substrate film may have an arithmetic mean surface roughness of 1 μm or less (e.g. about 0.05 μm to 0.75 μm, typically about 0.1 μm to 0.5 μm). In this description, the arithmetic mean surface roughness can be measured using a general surface roughness gauge (e.g. non-contact three-dimensional surface profilometer under model name WYKO NT-3300 available from Veeco).

When an adhesively single-faced PSA sheet is wound to bring the back face of the substrate film in contact with the PSA layer surface, the back face (opposite from the surface to be provided with a PSA layer) of the substrate film may be subjected as necessary to release treatment with a silicone-based, long chain alkyl-based, fluorine-based release agent or the like. The release treatment brings about effects such as easier unwinding of the PSA sheet wound in a roll. On the other hand, the PSA layer-side surface of the substrate film may be subjected to a heretofore known surface treatment such as corona discharge treatment and primer coating for purposes such as increasing the tightness of adhesion between the substrate and the PSA layer.

The thickness of the substrate film is not particularly limited and can be suitably selected in accordance with the purpose. In general, the substrate thickness is suitably 1 μm or larger (e.g. about 2 μm to 500 μm), or preferably about 5 μm to 500 μm (e.g. 10 μm to 200 μm, typically 15 μm to 100 μm). It is advantageous to limit the thickness of the substrate film in view of making the PSA sheet thinner, smaller, lighter, resources-saving, and so on.

When the substrate film comprises a foam film, the thickness of the foam-containing substrate (e.g. a foam substrate film) can be suitably selected in accordance with the strength and flexibility of the PSA sheet, intended purposes and so on. From the standpoint of the impact-absorbing properties, etc., the foam-containing substrate has a thickness of suitably 30 μm or larger, preferably 50 μm or larger, or more preferably 60 μm or larger (e.g. 80 μm or larger). From the standpoint of making the PSA sheet thinner, smaller, lighter, resource-saving, and so on, the thickness of the foam-containing substrate is usually suitably 1 mm or smaller. The use of the foam film disclosed herein can bring about excellent impact-absorbing capabilities even when the thickness is about 350 μm or smaller (more preferably 250 μm or smaller, e.g. 180 μm or smaller). The thickness of the foam film (possibly a foam layer) in the foam-containing substrate can also be preferably selected from the ranges exemplified as the thickness of the aforementioned foam-containing substrate.

<PSA Layer>

The PSA layer disclosed herein typically refers to a layer formed of a material (PSA) that exists as a soft solid (a viscoelastic material) in a room temperature range and has a property to adhere easily to adherend with some pressure applied. As defined in “Adhesion: Fundamental and Practice” by C. A. Dahlquist (McLaren & Sons (1966), P. 143), the PSA referred to herein is generally a material that has a property satisfying complex tensile modulus E*(1 Hz)<10⁷ dyne/cm² (typically, a material that exhibits the described characteristics at 25° C.).

The PSA layer disclosed herein may comprise, as its base polymer, one, two or more species among acrylic polymers, rubber-based polymers, polyester-based polymers, urethane-based polymers, polyether-based polymers, silicone-based polymers, polyamide -based polymers, fluorine-based polymers, etc. From the standpoint of the adhesive properties (e.g. peel strength, repulsion resistance), molecular design, etc., acrylic polymers can be preferably used. In other words, the PSA layer is preferably an acrylic PSA layer that comprises an acrylic polymer as its base polymer. The “base polymer” of a PSA refers to the primary component (typically, a component accounting for more than 50% by weight) among polymers in the PSA.

As the acrylic polymer, for example, a polymer of a monomeric starting material comprising an alkyl (meth)acrylate as a primary monomer and possibly comprising a secondary monomer copolymerizable with the primary monomer is preferable. The primary monomer herein refers to a component that accounts for higher than 50% by weight of the monomer composition in the monomeric starting material.

As the alkyl (meth)acrylate, for instance, a compound represented by the following formula (1) can preferably be used:

CH₂═C(R¹)COOR²   (1)

Herein, R¹ in the formula (1) is a hydrogen atom or a methyl group. R² is a acyclic alkyl group having 1 to 20 carbon atoms (hereinafter, such a numerical range of carbon atoms may be indicated as “C₁₋₂₀”). From the standpoint of the storage elastic modulus of the PSA, etc., an alkyl (meth)acrylate having a C₁₋₁₂ (e.g. C_(2-10,) typically C₄₋₈) acyclic alkyl group for R² is preferable. For the alkyl (meth)acrylate having a C₁₋₂₀ acyclic alkyl group for R², solely one species or a combination of two or more species can be used. Preferable alkyl (meth)acrylates include n-butyl acrylate and 2-ethylhexyl acrylate.

The secondary monomer copolymerizable with the alkyl (meth)acrylate as the primary monomer may be useful in introducing crosslinking points into the acrylic polymer and increasing the cohesive strength of the acrylic polymer. As the secondary monomer, one, two or more species can be used among functional group-containing monomers such as carboxy group-containing monomers, hydroxy group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, and monomers having nitrogen-containing rings. The secondary monomer may also be a vinyl ester-based monomer such as vinyl acetate, an aromatic vinyl compound such as styrene, a sulfonate group-containing monomer, a phosphate group-containing monomer, etc. For instance, from the standpoint of increasing the cohesive strength, an acrylic polymer in which a carboxy group-containing monomer or a hydroxy group-containing monomer is copolymerized as the secondary monomer is preferable. Preferable examples of the carboxy group-containing monomer include acrylic acid and methacrylic acid. Preferable examples of the hydroxy group-containing monomer include 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate.

The amount of the secondary monomer is suitably 0.5% by weight of all monomers in the acrylic polymer, or preferably 1% by weight or more. The amount of the secondary monomer is suitably 30% by weight or less of all the monomers, or preferably 10% by weight or less (e.g. 5% by weight or less). When a carboxy group-containing monomer is copolymerized in the acrylic polymer, from the standpoint of combining adhesive strength and cohesive strength, the carboxy group-containing monomer content is preferably within a range of about 0.1% to 10% by weight (e.g. 0.2% to 8% by weight, typically 0.5% to 5% by weight) of all the monomers used in the synthesis of the acrylic polymer. When a hydroxy group-containing monomer is copolymerized in the acrylic polymer, from the standpoint of combining adhesive strength and cohesive strength, the hydroxy group-containing monomer content is preferably within a range of about 0.001% to 10% by weight (e.g. 0.01% to 5%, typically 0.02% to 2% by weight) of all the monomers used in the synthesis of the acrylic polymer. When a vinyl ester-based monomer such as vinyl acetate is copolymerized as the secondary monomer, the vinyl ester-based monomer content is preferably about 30% by weight or less (typically 0.01% to 30% by weight, e.g. 0.1% to 10% by weight) of all the monomers used in the synthesis of the acrylic polymer.

The method for obtaining the acrylic polymer is not particularly limited. Various polymerization methods known as procedures for the synthesis of acrylic polymer can be suitably employed, such as solution polymerization, emulsion polymerization, bulk polymerization and suspension polymerization. It is also possible to employ active energy ray radiation polymerization which involves irradiation of UV etc. For instance, a desirable acrylic polymer can be obtained by dissolving or dispersing a monomer mixture in a suitable polymerization solvent (toluene, ethyl acetate, water, etc.) and carrying out polymerization using a polymerization initiator such as an azo-based polymerization initiator and a peroxide-based initiator.

From the standpoint of combining adhesive strength and cohesive strength in a well-balanced way, the acrylic polymer disclosed herein preferably has a weight average molecular weight (Mw) in a range of 10×10⁴ or higher, but 100×10⁴ or lower. An acrylic polymer whose Mw is 20×10⁴ or higher, but 70×10⁴ or lower (e.g. 30×10⁴ or higher, but 50×10⁴ or lower) may bring about better results. In this description, Mw refers to the value based on standard polystyrene obtained by GPC (gas permeation chromatography).

From the standpoint of increasing the cohesive strength, the PSA composition preferably comprises a crosslinking agent. The type of crosslinking agent is not particularly limited; one, two or more species can be suitably selected and used among heretofore known crosslinking agents. Preferable examples of the crosslinking agent include isocyanate-based crosslinking agents and epoxy-based crosslinking agents. The amount of the crosslinking agent used is not particularly limited. For instance, to 100 parts by weight of the acrylic polymer, it can be selected from a range of about 10 parts by weight or less (e.g. about 0.005 part to 10 parts by weight, preferably about 0.01 part to 5 parts by weight).

The PSA layer disclosed herein may have a composition comprising a tackifier. The tackifier is not particularly limited. Various tackifier resins can be used, such as rosin-based tackifier resin, terpene-based tackifier resin, hydrocarbon-based tackifier resin, epoxy-based tackifier resin, polyamide-based tackifier resin, elastomer-based tackifier resin, phenolic tackifier resin, and ketone-based tackifier resin. These tackifier resins can be used solely as one species or in a combination of two or more species.

The tackifier resin preferably has a softening point (temperature of softening) of about 60° C. or higher (preferably about 80° C. or higher, typically 100° C. or higher). By this, the PSA sheet can be obtained with higher adhesive strength. The upper limit of the softening point of the tackifier resin is not particularly limited; it can be about 180° C. or lower (e.g. about 140° C. or lower). The softening point of tackifier resin referred to herein is defined as the value measured by the softening point test method (ring and ball method) specified either in JIS K5902:2006 or in JIS K2207:2006.

The amount of tackifier resin can be suitably selected in accordance with the target adhesive properties (adhesive strength, etc.). For instance, by solid content, it is preferable to use a tackifier at a ratio of about 10 parts to 100 parts by weight (more preferably 20 parts to 80 parts by weight, or yet more preferably 30 parts to 60 parts by weight) relative to 100 parts by weight of the base polymer (preferably an acrylic polymer).

The PSA composition may comprise, as necessary, various additives generally known in the field of PSA compositions, such as leveling agent, crosslinking accelerator, plasticizer, softening agent, filler, anti-static agent, anti-aging agent, UV-absorbing agent, antioxidant and photo-stabilizing agent. With respect to these various additives, heretofore known species can be used by typical methods.

The PSA layer disclosed herein may be formed from aqueous, solvent-based, hot-melt, and active energy ray-curable types of PSA composition, etc. The aqueous PSA composition refers to a PSA composition in a form comprising PSA (PSA layer-forming components) in a solvent whose primary component is water (in an aqueous solvent), typically including a so-called water-dispersed PSA composition (a composition in a form where at least part of the PSA is dispersed in water). The solvent-based PSA composition refers to a PSA composition in a form comprising PSA in an organic solvent. From the standpoint of reducing environmental stress, an aqueous PSA composition is preferable. From the standpoint of the adhesive properties, etc., a solvent-based PSA composition is preferably used.

The PSA layer disclosed herein can be formed by a heretofore known method. For instance, a direct method can be preferably employed, in which a PSA composition is directly provided (typically applied) to a substrate and allowed to dry to form a PSA layer. Alternatively, a transfer method can also be used, in which a PSA composition is provided to a releasable surface (a release face) and allowed to dry to form a PSA layer on the surface and the PSA layer is transferred to a substrate. As the release face, a release liner surface, the back face of a substrate treated with a release agent, and the like can be used.

The PSA composition can be applied using a known or commonly used coater, such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, die coater, bar coater, knife coater, and spray coater. Alternatively, the PSA composition can be applied by immersion, curtain coating, etc.

From the standpoint of facilitating the crosslinking reaction, increasing the productivity, etc., the PSA composition is preferably heated to dry. The drying temperature can be, for instance, about 40° C. to 150° C., or usually preferably about 60° C. to 130° C. After dried, the PSA composition can be further allowed to age for adjustment of migration of the components in the PSA layer, for the progress of the crosslinking reaction, for releasing the distortion possibly present in the substrate and PSA layer, etc.

The thickness of the PSA layer disclosed herein is not particularly limited; it can be suitably selected in accordance with the purpose. Usually, from the standpoint of the productivity such as the drying efficiency, adhesive properties, etc., it is suitably about 0.5 μm to 200 μm, or preferably about 2 μm to 200 μm (e.g. 5 μm to 100 μm, typically 10 μm to 50 μm). It is advantageous to limit the thickness of the PSA layer in view of making the PSA sheet thinner, smaller, lighter, resource-saving, and so on. When the art disclosed herein is implemented in an embodiment of an adhesively double-faced sheet having a PSA layer on each face of a substrate, the thicknesses of the respective PSA layers can be the same or different.

<Coating Layer>

The coating layer partially covering the PSA layer surface is not particularly limited as long as it can provide air release properties. A favorable example of the coating layer material is a resin material. From the standpoint of the appearance, the coating layer is preferably formed from a transparent or semi-transparent resin material.

Examples of the resin material form which the coating layer can be formed include a polyurethane-based resin, a phenolic resin, an epoxy-based resin, a polyamide-based resin, a urea melamine-based resin, a silicone-based resin, a polysilazane-based resin, a fluororesin, a phenoxy resin, a methacrylic resin, an acrylic resin, an acrylic urethane-based resin, an acrylic styrene-based resin, a polyarylate resin, a polyester-based resin, a polyolefinic resin, a polystyrene-based resin, polyvinyl chloride, a vinyl chloride/vinyl acetate copolymer, polyvinyl acetate, polyvinylidene chloride, polycarbonate, a cellulose, and a polyacetal. The resin can be one, two or more species of resin selected from various types of resins including a heat-curable resin, a UV-curable resin, an electron beam-curable resin, and a two-component resin that is curable upon mixing

The coating layer disclosed herein may comprise as necessary various additives such as fillers, anti-aging agent, antioxidant, UV absorber, crosslinking agent, slip agent, colorant (pigment, dye, etc.), antistatic agent, viscosity-adjusting agent (thixotropic agent, thickening agent, etc.), and film-forming aid.

The coating layer is preferably non-adhesive or weakly adhesive. This preferably brings about good air release properties. Here, that the coating layer is non-adhesive or weakly adhesive means that the coating layer has a 180° peel strength less than 3 N/25 mm (typically less than 1 N/25 mm, including unmeasurably low adhesive strength). In particular, the 180° peel strength of the coating layer is determined by the following method: The PSA sheet having a coating layer over the entire PSA layer surface is cut to a 25 mm wide by 100 mm long size to obtain a measurement sample; in an environment at 23° C., 50% RH, the measurement sample is press-bonded over its coating layer surface to the surface of a stainless steel plate (SUS304BA plate) with a 2 kg roller moved back and forth once. If it does not adhere, it is considered non-adhesive here. The resultant is left standing in the same environment for 30 minutes. Using a universal tensile/compression tester, based on JIS Z 0237:2000, it is then measured for peel strength (N/20 mm) at a tensile speed of 300 mm/min at a peel angle of 180°.

The method for placing the coating layer on the PSA layer surface is not particularly limited. For instance, a method as described next can be employed. In particular, a coating layer-forming composition is prepared as necessary by dissolution or dispersion in a suitable solvent. Subsequently, by employing a suitable method among various known or commonly-used printing methods, the composition is provided to a releasable surface of a releasable support (or a coating layer-transferring film, typically a release liner) and allowed to cure. The releasable support surface on which the coating layer is formed is brought into contact with the PSA layer surface to transfer the coating layer onto the PSA layer surface. The coating layer is thus partially placed on the PSA layer surface. For instance, a desirable coating layer pattern such as a lattice pattern can be preferably formed by employing a method such as offset printing, silk screen printing, letterpress printing, flexographic printing, gravure printing, and inkjet printing. From the standpoint of the air release properties, gravure printing is more preferable. Alternatively, the coating layer-forming composition can be directly provided to the PSA layer surface and allowed to cure by means of, for instance, UV curing to place a coating layer partially on the PSA layer surface.

The thickness of the coating layer can be designed to obtain desirable air release properties and is not particularly limited. From the standpoint of combining air release properties and appearance, it is preferably up to about a half (e.g. up to one-third, typically up to one-fifth) the thickness of the PSA layer. In particular, from the standpoint of the air release properties, productivity, etc., the thickness of the coating layer is preferably 0.1 μm or greater (e.g. 0.5 μm or greater, typically 1 μm or greater). From the standpoint of the appearance, the coating layer has a thickness of preferably 10 μm or less, or more preferably 5 μm or less (e.g. 3 μm or less, typically 2 μm or less). The thickness of the coating layer can be obtained by TEM (transmission electron microscopy) analysis of a cross section of the PSA sheet.

The art disclosed herein is preferably implemented in an embodiment of a release liner-backed PSA sheet that has a release liner protecting the adhesive surface of the PSA sheet. As the release liner, any conventional release paper or the like can be used without any particular limitations. For example, a release liner having a release layer on a surface of a liner substrate such as resin film (PET, etc.) and paper; a release liner formed from a poorly-adhesive material such as a fluorine-based polymer (polytetrafluoroethylene, etc.) or a polyolefin-based resin (PE, PP, etc.); or the like can be used. The release layer can be formed, for instance, by subjecting the liner substrate to a surface treatment with a release agent such as a silicone-based, a long-chain alkyl-based, a fluorine-based, a molybdenum disulfide-based release agent or the like.

In a preferable embodiment, the release surface (on the side that makes contact with the PSA sheet's adhesive surface) of the release liner (release film) is formed smooth. A good coating layer transfer can be obtained when a coating layer is partially formed on the releasable surface of such a release liner and the release liner surface with the coating layer partially formed thereon is brought into contact with a PSA layer to transfer the coating layer onto the PSA layer surface (in this case, the release liner also serves as the coating layer-transferring film (release film)) The smooth texture of the surface of the release liner is transferred onto the PSA layer surface, whereby the adhesive properties tend to increase as well. Similar effects can be obtained by partially forming a coating layer on the releasable surface of such a release liner and further forming a PSA layer on the releasable surface with the coating layer formed thereon so as to cover the coating layer. In this case, the PSA layer formed is transferred to a surface of a substrate film. For these reasons, it is preferable that the release surface of the release liner has an arithmetic average surface roughness of 1 μm or less (e.g. about 0.05 μm to 0.75 μm, typically about 0.1 μm to 0.5 μm).

As shown in FIG. 5, the release liner used in fabricating the PSA sheet disclosed herein may be a coating layer-bearing release liner 110 that comprises a releasable support 20 having a releasable surface 120A. The releasable support 120 may have a release layer at least on one face of a liner substrate, or it can be a support formed from a low-adhesive material. The releasable surface 120A of the releasable support 120 is provided with a coating layer 30 that can be transferred to a PSA sheet. In other words, the coating layer 30 is arranged on the releasable surface 120A in a state that it can be separated by an adhesive strength of PSA, etc. With the use of such release liner 110 having a transferrable coating layer 30 on the surface, the PSA sheet disclosed herein is preferably fabricated. The features (shape, arrangement, relative position, size, pattern, etc.) of the coating layer provided to the releasable surface of the releasable support are basically the same as the features of the coating layer partially covering the PSA layer surface of the PSA sheet described earlier. Accordingly, the constructions of the coating layer-bearing area and the coating layer-free area as well as their relation (including the % surface area of the coating layer-free area) on the releasable surface are also basically the same as the constructions of the coating layer-bearing area and the coating layer-free area as well as their relation (including the % surface area of the coating layer-free area) on the PSA layer surface. Thus, details are omitted.

The thickness (overall thickness) of the release liner is not particularly limited. From the standpoint of the ease of removal, handling properties, strength, etc., it is preferably about 10 μm to 500 μm (e.g. 15 μm to 100 μm).

As described above, in applying the PSA sheet disclosed herein to an adherend, the sort of bubble formation can be efficiently prevented at the interface with the adherend. Thus, in either application method between application by hand (manual application) and application with an automated applicator or the like (automated application), the ease of application will improve. For example, when applied by manual application, the degree of dependence on skills of individuals can be reduced, thereby bringing about advantages such as increases in efficiency and quality of the application and their stabilization. When applied by automated application, failures during application such as trapping of bubbles and reapplication work can be reduced. Accordingly, either by manual application or by automated application, it is possible to bring about increases in application efficiency and quality, stabilization of the quality and so on, thereby increasing the productivity and quality of products made with the use of the PSA sheet as well. The art disclosed herein can bring about more uniform application; and therefore, it is particularly favorable as a PSA sheet that is applied with an automated applicator.

Between the PSA sheet and the adherend, the sort of bubble formation may occur, not just during the application, but also after the application as the time passes. In typical, after the PSA sheet is applied, upon storage and use in an environment at a relatively high temperature (e.g. 35° C. or higher), etc., aforementioned bubbles and the like may form between the PSA sheet and the adherend, causing degradation of the appearance. For instance, such high temperature conditions are likely to be reached in factories and outdoor in summer, inside electronics, etc. According to the art disclosed herein, even when used for applications exposed to such high temperature environments, the sort of bubble formation can be prevented.

With the benefit of the features described above, the PSA sheet disclosed herein can be preferably used for application to surfaces of various articles. In a preferable embodiment, it can be used as decorative sheets and surface protection sheets of various kinds, a fixing sheet for printing plates of flexographic printing and the like, a light-blocking sheet, and so on. For instance, it is preferable as a decorative sheet (typically a paint-substitute sheet) applied to vehicle exteriors, house building materials, and so on. It is also preferable for use inside electronics such as TVs as a cover sheet used to increase the smoothness of the outer face of a chassis or to cover uneven places such as of screw holes in surfaces of various parts. The use of such a cover sheet can decrease unevenness of the appearance of the adherend's outer surface and make the dimensional precision uniform. It can also be preferably used as an exterior sheet for battery packs for which the appearance is important.

Even when made thin, with the PSA sheet disclosed herein, it is possible to prevent degradation of appearance quality after its application while maintaining good adhesive properties. Thus, it can be preferably used for applications (e.g. for mobile electronics) where a thinner build and a lighter weight are required desirably with saving of resources. In particular, it can be preferably used as a surface protection sheet for mobile electronics (e.g. mobile phones, smartphones, tablet PCs, notebook PCs); for bonding/fixing applications in liquid crystal displays of these mobile electronics; for fixing protection panels (lenses) to protect the displays of these mobile electronics; for fixing key modules of mobile phones; and for like purposes. When used for the mobile electronics, the PSA sheet may have a shape in accordance with the purpose and so on, such as a frame shape and a ribbon shape (a strip shape). In this description, to be “mobile,” it is not sufficient that it can be just carried, but it needs to be mobile enough for an individual (an average adult) to be able to carry it by hand relatively easily.

Matters disclosed by this description include the following:

-   (1) A PSA sheet comprising a substrate film and a PSA layer provided     to at least one face of the substrate film, wherein

the PSA sheet further comprises a coating layer that partially covers the PSA layer surface, whereby the PSA layer surface has a coating layer-bearing area and a coating layer-free area,

the coating layer-free area occupies 70% or more of the PSA layer surface,

in a top view of the PSA layer surface, the coating layer-bearing area has a linearly extending part that runs from one edge to another edge of the PSA layer,

the linearly extending part has a width in a range of 0.1 mm to 2 mm

-   (2) The PSA sheet according to (1) above, wherein the PSA layer     surface is provided with a plurality of the linearly extending parts     among which the linearly extending parts in a group are placed at     intervals arranged in the width direction, whereby the coating     layer-bearing area has a stripe pattern -   (3) The PSA sheet according to (2) above, wherein the coating     layer-bearing area comprises a first stripe pattern and a second     stripe pattern that is placed to intersect the first stripe pattern,     whereby the coating layer-bearing area has a lattice pattern. -   (4) The PSA sheet according to (2) or (3) above, wherein, in the     coating layer-bearing area, the intervals between the plurality of     the linearly extending parts forming the stripe pattern are in a     range of 1.0 mm to 10 mm. -   (5) The PSA sheet according to any one of (1) to (4) above, having     an adhesive face on which the coating layer is partially placed, the     adhesive face exhibiting a 180° peel strength after 24-hour adhesion     of 13 N/25 mm or greater. -   (6) The PSA sheet according to any one of (1) to (5) above, wherein     the PSA layer comprises an acrylic polymer that accounts for more     than 50% (by weight) of all polymers in the PSA layer, with the     acrylic polymer comprising, as a monomer, more than 50% (by weight)     alkyl (meth)acrylate represented by a formula (1):

CH₂═C(R¹)COOR²   (1)

(in the formula (1), R¹ is a hydrogen atom or a methyl group; R² is an acyclic alkyl group with 1 to 20 carbon atoms).

-   (7) The PSA sheet according to (6) above, wherein the alkyl     (meth)acrylate is n-butyl acrylate and/or 2-ethylhexyl acrylate. -   (8) The PSA sheet according to (6) or (7) above, wherein the acrylic     polymer is an acrylic polymer in which a carboxy group-containing     monomer and/or a hydroxy group-containing monomer is copolymerized     as secondary monomer(s). -   (9) The PSA sheet according to any one of (6) to (8) above, wherein     the acrylic polymer is an acrylic polymer in which at least one     species of secondary monomer selected from the group consisting of     acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate and     4-hydroxybutyl acrylate is copolymerized. -   (10) The PSA sheet according to any one of (6) to (9) above, wherein     a carboxy group-containing monomer is copolymerized in the acrylic     polymer, the copolymerization ratio of the carboxy group-containing     monomer being 0.1% to 10% (by weight) to all the monomers used in     synthesizing the acrylic polymer. -   (11) The PSA sheet according to any one of (6) to (10) above,     wherein a hydroxy group-containing monomer is copolymerized in the     acrylic polymer, the copolymerization ratio of the hydroxy     group-containing monomer being 0.001% to 10% (by weight) to all the     monomers used in synthesizing the acrylic polymer. -   (12) The PSA sheet according to any one of (6) to (11) above,     wherein the PSA layer comprises an isocyanate-based crosslinking     agent and/or an epoxy-based crosslinking agent. -   (13) The PSA sheet according to any one of (6) to (12) above,     wherein the PSA layer comprises a tackifier resin with a softening     point of 100° C. to 140° C. in an amount of 30 parts to 60 parts by     weight to 100 parts by weight of the acrylic polymer, the tackifier     resin being at least one species selected from the group consisting     of a rosin-based tackifier resin, a terpene-based tackifier resin     and a hydrocarbon-based tackifier resin. -   (14) The PSA sheet according to any one of (1) to (13) above,     wherein the coating layer comprises a transparent resin. -   (15) The PSA sheet according to any one of (1) to (14) above,     wherein the coating layer comprises at least one species of resins     selected from the group consisting of a polyurethane-based resin, a     phenolic resin, an epoxy-based resin, a polyamide-based resin, a     urea melamine-based resin, a silicone-based resin, a     polysilazane-based resin, a fluororesin, a phenoxy resin, a     methacrylic resin, an acrylic resin, an acrylic urethane-based     resin, an acrylic styrene-based resin, a polyarylate resin, a     polyester-based resin, a polyolefinic resin, a polystyrene-based     resin, polyvinyl chloride, a vinyl chloride/vinyl acetate copolymer,     polyvinyl acetate, polyvinylidene chloride, polycarbonate, a     cellulose, and a polyacetal. -   (16) The PSA sheet according to any one of (1) to (15) above,     wherein the coating layer is of a polyurethane-based resin. -   (17) The PSA sheet according to any one of (1) to (16) above,     wherein the coating layer is of a two-component polyurethane-based     resin that is curable upon mixing -   (18) The PSA sheet according to any one of (1) to (17) above,     wherein the substrate film is a polyolefinic resin film, a     polyester-based resin film, a vinyl chloride-based resin film, a     vinyl acetate-based resin film, a polyimide-based resin film, a     polyamide-based resin film, a fluororesin film, or a cellophane     film. -   (19) The PSA sheet according to any one of (1) to (18) above,     wherein the substrate film is a polyester film. -   (20) The PSA sheet according to any one of (1) to (19) above,     wherein the substrate film is a polyethylene terephthalate film. -   (21) The PSA sheet according to any one of (1) to (20) above,     wherein the substrate film exhibits a total light transmittance of     80% or higher. -   (22) The PSA sheet according to any one of (1) to (17) above,     wherein the substrate film is a foam film. -   (23) The PSA sheet according to (22) above, wherein the foam film     has a mean pore diameter of 10 μm to 200 μm and a density of 0.01     g/cm³ to 0.7 g/cm³. -   (24) The PSA sheet according to (22) or (23) above, wherein the foam     film is an acrylic resin foam or a polyolefmic resin foam. -   (25) A release liner-backed PSA sheet comprising the PSA sheet     according to any one of (1) to (24) above and a release liner that     protects the adhesive face of the PSA sheet, wherein the release     liner has a smooth surface on the adhesive face side. -   (26) A release liner for a PSA sheet, the release liner comprising a     releasable support having at least one releasable face, wherein

the releasable face of the releasable support is provided with a coating layer disclosed herein that can be transferred onto a PSA sheet, whereby the releasable surface has a coating layer-bearing area and a coating layer-free area,

the coating layer-free area occupies 70% or more of the PSA layer surface,

the coating layer has a linearly extending part that runs from one edge to another edge of the releasable surface, and

the linearly extending part has a width in a range of 0.1 mm to 2 mm

Several Examples related to the present invention are described below, but the present invention is not intended to be limited to these Examples. In the description below, “parts” and “%” are by weight unless otherwise noted.

<<Experiment 1>> EXAMPLE 1A (Preparation of PSA Composition)

In a reaction vessel equipped with a stirrer, thermometer, nitrogen inlet, reflux condenser and addition funnel, were placed 70 parts of n-butyl acrylate, 30 parts of 2-ethylhexyl acrylate, 3 parts of acrylic acid, 0.05 part of 4-hydroxybutyl acrylate, 0.08 part of azobisisobutyronitrile as polymerization initiator and toluene as the polymerization solvent. Solution polymerization was carried out at 60° C. for 6 hours to obtain an acrylic polymer solution in toluene (viscosity 28 Pa·s, 40% non-volatiles). The resulting acrylic polymer had a Mw of about 44×10⁴.

To 100 parts of the acrylic polymer in the toluene solution, was admixed 30 parts of a polymerized rosin pentaerythritol ester (trade name PENSEL D125 available from Arakawa Chemical Industries, Ltd.; softening point 125° C.) followed by 3 parts of an isocyanate-based crosslinking agent (trade name CORONATE L available from Nippon Polyurethane Industry Co., Ltd.) to prepare an acrylic PSA composition.

(Formation of Coating Layer)

A coating layer-forming material (urethane-based, two-component ink (curable when mixed)) was gravure-printed on the releasable face of 75 μm thick release film (trade name FMN-75WD (C1·CA1) available from Fujiko Co., Ltd.) to form a coating layer (coating thickness 1.5 μm, transparent) in a lattice pattern.

(Fabrication of PSA Sheets)

A 75 μm thick PET substrate film (trade name LUMIRROR available from Toray Industries, Inc.) was obtained. Onto the corona discharge-treated face of the PET substrate, the PSA composition was applied to a thickness of 15 μm (after dried) and allowed to dry at 100° C. for 3 minutes to form a PSA layer on one face of the PET substrate film.

The coating layer-bearing release film obtained above was brought into contact with the surface of the PSA layer to transfer the coating layer onto the PSA layer surface. The release film was used as it was to protect the PSA layer surface. A PSA sheet according to this Example was thus fabricated. The PSA layer surface of this PSA sheet had a lattice pattern formed of the coating layer as shown in FIG. 1, with the long parts of the coating layer-bearing area having a width (line width) of 2 mm and intervals of 8 mm

EXAMPLE 1B

The intervals between the long parts of the coating layer-bearing area were changed to 13 mm. Otherwise in the same manner as Example 1A, a PSA sheet according to this Example was fabricated.

EXAMPLE 1C

Without a coating layer, but otherwise in the same manner as Example 1A, a PSA sheet according to this Example was fabricated.

[180° Peel Strength]

With respect to the PSA sheet according to each Example, the to-SUS 180° peel strength (adhesive strength) was tested. In particular, the PSA sheet was cut to a 25 mm by 100 mm long size to obtain a measurement sample. In an environment at 23° C., 50% RH, the adhesive surface of the measurement sample was press-bonded to the surface of a stainless steel plate (SUS 304BA plate) with a 2 kg roller moved back and forth once. This was left standing in the same environment for 30 minutes, 24 hours, or 72 hours. Subsequently, using a universal tensile/compression tester, based on JIS Z 0237:2000, the peel strength (N/25 mm) was determined at a tensile speed of 300 mm/min at a peel angle of 180°. Three measurements were carried out for each Example under each condition (standing time after press-bonded) and their average value was used. With the 180° peel strength value of Example 1C (with the coating layer-free area occupying 100% surface area) being 100%, the 180° peel strength values of Examples 1A and 1B were shown as the relative values (%) and the decreases in adhesive strength were evaluated in association with the reduced surface areas of their coating layer-bearing areas. The results are shown in Table 1 with the % surface area of the coating layer-bearing areas.

TABLE 1 Ex. 1A Ex. 1B Ex. 1C % Surface area of coating layer-free area 64 75 100 Adhesive strength After 30 min (initial) (N/25 mm) 11.4 13.5 20.9 (%) 55 65 100 After 24 hours (N/25 mm) 12.0 17.2 22.0 (%) 55 78 100 After 72 hours (N/25 mm) 13.4 17.8 23.9 (%) 56 74 100

As shown in Table 1, with increasing surface area of the coating layer and decreasing exposed surface area of the PSA layer, the initial adhesive strength (at 30 minutes after adhered to the adherend) decreased in a proportional manner. On the other hand, with respect to the long-term adhesive strength (at 24 hours or 72 hours after adhered to the adherend), in Example 1A where the coating layer-free area occupied 64% surface area, the adhesive strength did not increase with aging, impairing the proportional relationship of the initial adhesive strength and showing a value that was significantly lower than the value expected from the results of Examples 1B and 1C in which the coating layer-free areas occupied 75% and 100% surface areas, respectively. These results indicate that in an embodiment where the coating layer is partially placed on the PSA layer surface, to obtain high long-term adhesive strength, the coating layer-free area needs to occupy 70% or more surface area.

<<Experiment 2>> EXAMPLES 2A-2J

The width (mm) and intervals (mm) of long parts of the coating layer-bearing area were set to the values shown in Table 2. By the same method as for Example 1A, a PSA sheet according to each Example was fabricated. Table 2 shows the 24-hour adhesion strength (N/25 mm) determined for each Example. The measurement method is as described above. It is noted that Examples 2A, 2I and 2J had the same constructions as Examples 1B, 1A and 1C.

[Air Release Properties]

With respect to the PSA sheet according to each Example, the degree of elimination of air upon application to the smooth surface of a transparent adherend was visually inspected and graded into the four grades shown below. The results are shown in Table 2.

-   A: Excellent air release properties were shown with no bubbles left     between the PSA sheet and the adherend. -   B: Bubbles between the PSA sheet and the adherend were eliminated     relatively quickly, leaving no bubbles. -   C: Bubbles between the PSA sheet and the adherend were eliminated     relatively slowly, leaving no bubbles. -   D: Bubbles were left between the PSA sheet and the adherend.

TABLE 2 Ex. 2A Ex. 2B Ex. 2C Ex. 2D Ex. 2E Ex. 2F Ex. 2G Ex. 2H Ex. 2I Ex. 2J Width (mm) 2 1.5 1.0 0.5 0.3 0.2 0.2 0.1 2 0 Interval (mm) 13 8.5 9.0 9.5 2.7 2.8 1.8 1.4 8 0 % Surface area of 75 72 81 90 81 87 81 87 64 100 coating layer- free area 24-h Adhesion 17.2 14.5 16.3 19.3 14.7 18.2 16.0 18.1 12.0 22.0 strength (N/25 mm) Air release properties A A A A A B B C A D

As shown in Table 2, the PSA sheets according to Examples 2A to 2H (each with the coating layer-free area occupying 70% or more of the PSA layer surface and the long parts of the coating layer-bearing area having a width in the range of 0.1 mm to 2 mm) showed high long-term adhesive strength (24-hour adhesion strength) and good air release properties at the interfaces between the PSA sheets and the adherend. With respect to the PSA sheets according to Examples 2C to 2H, after adhered to the adherend, their coating layers were hardly visible when seen from the adherend side and from the PSA sheet side, making great appearances. The PSA sheets according to Examples 2C to 2G had excellent appearances and also combined well-balanced high long-term adhesive strength and air release properties. Among them, Examples 2E, 2F and 2G had particularly great appearances. On the other hand, Example 2I with the coating layer-free area occupying less than 70% surface area showed lower long-term adhesive strength as compared to Examples 2A to 2H. In Example 2J with no coating layer, bubbles were left between the PSA sheet and the adherend.

Although specific embodiments of the present invention have been described in detail above, these are merely for illustrations and do not limit the scope of claims. The art according to the claims includes various modifications and changes made to the specific embodiments illustrated above.

REFERENCE SIGNS LIST

-   1, 2 PSA sheets -   1A adhesive surface -   10 substrate film -   20, 21, 22 PSA layers -   20A PSA layer surface -   30 coating layer -   40, 41, 42 release liners -   50 coating layer-bearing area -   51 coating layer-free area -   60 coating layer pattern (lattice pattern) -   70 first stripe pattern -   75A, 75B, 75C long parts (linearly extending parts) -   80 second stripe pattern -   85A, 85B, 85C long parts (linearly extending parts) -   110 coating layer-bearing release liner -   120 releasable support -   120A releasable face 

1. A pressure-sensitive adhesive sheet comprising a substrate film and a pressure-sensitive adhesive layer provided to at least one face of the substrate film, wherein the pressure-sensitive adhesive sheet further comprises a coating layer that partially covers the pressure-sensitive adhesive layer surface, whereby the pressure-sensitive adhesive layer surface has a coating layer-bearing area and a coating layer-free area, the coating layer-free area occupies 70% or more of the pressure-sensitive adhesive layer surface, in a top view of the pressure-sensitive adhesive layer surface, the coating layer-bearing area has a linearly extending part that runs from one edge to another edge of the pressure-sensitive adhesive layer, the linearly extending part has a width in a range of 0.1 mm to 2 mm.
 2. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive layer surface is provided with a plurality of the linearly extending parts among which the linearly extending parts in a group are placed at intervals arranged in the width direction, whereby the coating layer-bearing area has a stripe pattern.
 3. The pressure-sensitive adhesive sheet according to claim 2, wherein the coating layer-bearing area comprises a first stripe pattern and a second stripe pattern that is placed to intersect the first stripe pattern, whereby the coating layer-bearing area has a lattice pattern.
 4. The pressure-sensitive adhesive sheet according to claim 2, wherein, in the coating layer-bearing area, the intervals between the plurality of the linearly extending parts forming the stripe pattern are in a range of 1.0 mm to 10 mm.
 5. The pressure-sensitive adhesive sheet according to claim 1, having an adhesive surface on which the coating layer is partially placed, the adhesive face exhibiting a 180° peel strength after 24-hour adhesion of 13 N/25 mm or greater.
 6. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive layer comprises an acrylic polymer that accounts for more than 50% by weight of all polymers in the pressure-sensitive adhesive layer, with the acrylic polymer comprising, as a monomer, more than 50% by weight alkyl (meth)acrylate represented by a formula (1): CH₂═C(R¹)COOR²   (1) wherein R¹ is a hydrogen atom or a methyl group; R² is an acyclic alkyl group with 1 to 20 carbon atoms. 7-9. (canceled)
 10. The pressure-sensitive adhesive sheet according to claim 6, wherein a carboxy group-containing monomer is copolymerized in the acrylic polymer, the copolymerization ratio of the carboxy group-containing monomer being 0.1% to 10% by weight to all monomers used in the synthesis of the acrylic polymer.
 11. The pressure-sensitive adhesive sheet according to claim 6, wherein a hydroxy group-containing monomer is copolymerized in the acrylic polymer, the copolymerization ratio of the hydroxy group-containing monomer being 0.001% to 10% by weight to all monomers used in the synthesis of the acrylic polymer.
 12. The pressure-sensitive adhesive sheet according to claim 6, wherein the pressure-sensitive adhesive layer comprises an isocyanate-based crosslinking agent and/or an epoxy-based crosslinking agent.
 13. The pressure-sensitive adhesive sheet according to claim 6, wherein the pressure-sensitive adhesive layer comprises a tackifier resin with a softening point of 100° C. to 140° C. in an amount of 30 parts to 60 parts by weight to 100 parts by weight of the acrylic polymer, and the tackifier resin is at least one species selected from the group consisting of a rosin-based tackifier resin, a terpene-based tackifier resin and a hydrocarbon-based tackifier resin.
 14. The pressure-sensitive adhesive sheet according to claim 1, wherein the coating layer comprises a transparent resin.
 15. (canceled)
 16. The pressure-sensitive adhesive sheet according to claim 1, wherein the coating layer is of a polyurethane-based resin.
 17. The pressure-sensitive adhesive sheet according to claim 1, wherein the coating layer is of a two-component polyurethane-based resin that is curable upon mixing.
 18. (canceled)
 19. The pressure-sensitive adhesive sheet according to claim 1, wherein the substrate film is a polyester film.
 20. (canceled)
 21. The pressure-sensitive adhesive sheet according to claim 1, wherein the substrate film exhibits a total light transmittance of 80% or higher.
 22. The pressure-sensitive adhesive sheet according to claim 1, wherein the substrate film is a foam film.
 23. The pressure-sensitive adhesive sheet according to claim 22, wherein the foam film has a mean pore diameter of 10 p.m to 200 p.m and a density of 0.01 g/cm³ to 0.7 g/cm³.
 24. The pressure-sensitive adhesive sheet according to claim 22, wherein the foam film is an acrylic resin foam or a polyolefinic resin foam.
 25. A release liner-backed pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive sheet according to claim 1, and a release liner that protects the adhesive face of the pressure-sensitive adhesive sheet wherein the release liner has a smooth surface on the adhesive face side.
 26. A release liner for a pressure-sensitive adhesive sheet, the release liner comprising a releasable support having at least one releasable face, wherein the releasable face of the releasable support is provided with a coating layer capable of being transferred onto the pressure-sensitive adhesive sheet, whereby the releasable surface has a coating layer-bearing area and a coating layer-free area, the coating layer-free area occupies 70% or more of the pressure-sensitive adhesive layer surface, the coating layer has a linearly extending part that runs from one edge to another edge of the releasable face, the linearly extending part has a width in a range of 0.1 mm to 2 mm. 